水轮机内部上下游涡流时空演化和相互作用的流动机理研究

With the increase in the penetration of the electricity generated from variable renewable energy sources, Francis turbines are more required to enhance their flexibility by extending the operating range, which has raised significant concern about instabilities caused by the unsteady flow in turbine during off-design operation. The current study aims to investigate the characteristics of unsteady flow field with emphasis on the interaction of vortex rope in the draft and vortical flows in the upstream. Firstly, the simulation method coupled with the influence of compressibility of water is studied based on the open access resource about a high-head Francis turbine test case. Then the pressure fluctuations triggered by the vortex rope is decomposed into relevant patterns. The space-time propagation mechanism of these patterns will be studied in order to understand which part of the fluctuation may be transposable to full scale machine and the actual relations with the upstream instabilities. Secondly, by taking advantages of several vortex dynamics methods, the typical vortical flow structures and their characteristic parameters will be recognized, allowing the quantitative analysis of the dynamic evolution history of the swirling flows and their relationship with the corresponding pressure fluctuations. Finally, the theoretical analysis model of vortex rope is established to clarify the influences of the upstream and downstream instabilities. All works make considerable efforts to illustrate the flow mechanism of unsteady swirling flows in Francis turbine with the influence of upstream and downstream interactions, which could eventually provide theoretical support for the safety of Francis turbine with extended operating range.

随着可再生能源不断引入电网，以混流式水轮机为代表的水电机组亟需扩大运行范围，然而偏工况下水轮机内部非稳态流动及其诱发的不稳定问题是最主要的制约因素。本项目针对尾水管涡带与上游不稳定涡流相互作用的流动机理开展研究。首先基于公开的模型试验算例，开展考虑流体弱压缩性的数值计算方法研究。然后对尾水管涡带诱发的压力脉动进行多模式分解，研究不同模式的时空传播规律，建立其与上游不稳定现象的关联关系；其次，利用多种涡动力学方法，准确识别非稳态流场结构及其特征参数，定量研究它们的演化规律，及伴随其产生的压力脉动响应特性；最后，构建反映上下游耦合关系的尾水管涡带理论分析模型，进行稳定性诊断并评估上下游的相互影响作用。本项目的顺利开展将揭示偏工况流场内不稳定涡流的时空演化和上下游相互作用对运行稳定性的影响，为进一步扩大水轮机安全运行范围提供理论指导。

在我国“双碳”目标的战略背景下，以水轮机为核心的水力机组亟需进一步扩大安全稳定运行范围。本项目针对偏工况下混流式水轮机内部非稳态涡流的时空演化、相互作用的流动机理，及伴随其产生的压力脉动响应特性开展了研究。首先构建了适合复杂涡流流场的数值计算方法为整个研究奠定基础。对比研究不同涡识别方法的适用性和特点，并对目标涡流结构进行可视化研究，提取不同工况下涡流结构的特征参数，阐明了不同工况下水轮机特征涡流在全流域的产生-发展-溃灭等演化规律。通过研究尾水管涡带诱发压力脉动的同步/异步模式信号，揭示了负荷变化对不同模式压力脉动特征的影响及其在上下游的传播规律。基于本征正交分解（POD）方法对尾水管内复杂流场进行解耦降阶，明晰了各阶涡流结构的位置、强度和构成。基于经验模态分解（EMD）和指数能量法，对尾水管压力脉动信号进行特征提取，获得信号的能量分布规律。构建能够准确反映尾水管涡流强度的特征向量，通过识别不同工况下流态样本的不稳定程度，进一步发展了偏工况下尾水管流场的理论分析方法和诊断方法。此外，不稳定涡流容易诱发空化从而造成更严重的不稳定问题，因此项目对空化现象也进行了一系列探索研究。本项目成果可为混流式水轮机的优化设计及电站的智能预警系统提供理论依据。